Re: [PATCH net v6] usbnet: fix smp_processor_id() use in preemptible context

On Thu, Oct 08, 2026 at 09:43:38AM -0700, Jakub Kicinski wrote:

On Thu, 8 Oct 2026 10:16:02 -0400 Alan Stern wrote:

quoted

quoted

This looks odd, how did we miss this for 8 years.

Greg is probably busy, but would be good to get a confirmation
from either him or some other USB expert that the callbacks
can indeed be called in process context.  

They can be called in BH context with interrupts enabled.  Is that close 
enough?

BH should be fine on !RT. The patch, AFAIU, is because vhci calls
the completion callbacks in pure, unadulterated process context.
I'm trying to figure out how urgent the fix is, basically.
If it's a vhci bug then the usbnet fix is at most an RT problem.
Also if vhci is doing something wrong we don't want a truckload
of slop patches sent our way to fix 300 drivers :S

As far as I am aware, there aren't really any guarantees on the 
context of a USB URB-completion callback.  The kerneldoc for struct urb 
in include/linux/usb.h says "The completion callback is made 
in_interrupt()", but that is most definitely out of date.

Drivers shouldn't rely on any particular context guarantees.  Not even 
whether local irqs are enabled/disabled.

Alan Stern